Antenna system



Jan. 3, 1950 G. H. BAKER ETAL ANTENNA SYSTEM 2 Sheets-Sheet l Filed June13, 1946 wm. mm.

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ATTORNEY Jan. 3, 1950 G. H. BAKER ETAL ANTENNA SYSTEM Filed June l5,1946 2 Sheets-Sheet 2 N .Sk

Patented Jan. 3, 1950 UNITED STATES PATENT OFFICE ANTENNA SYSTEMApplication June 13, 1946, Serial No. 676,393

4 Claims. (Cl. Z50-33.65)

This invention relates to transmission systems and to antenna apparatusdesigned to operate therewith. It is more particularly related to meansfor eliminating, in directive radio systems, false directionindications.

As is known, because of the fineness of resolution required in radar orreflection type target locating systems, it is customary to employdecimetric or centimetric radio waves and a highly directive antennasystem. The magnetron oscillators ordinarily utilized for generating theaforementioned waves are often affected by impedance changes in theattached load and, when the variation in the load impedance is ofsuiicient magnitude, the power output or the frequency of oscillation ofthe magnetron oscillator will be changed. A shift in oscillatingfrequency does not greatly affect the transmitting operation, but whenthe reflected energy of shifted frequency is applied to the sharplytuned receiving circuit, it is subjected to frequency discrimination dueto the filter action in the heterodyne type receiver which actionintroduces directional errors. It is of course highly desirable toeliminate all causes producing the above-mentioned, and other,directional errors.

Also, because of the flneness of resolution usual- 1y required in suchsystems, various arrangements have been employed to sharpen theirdirection indicating properties. Since direction is often determined bypointing a beam, or a directive lobe, at the object to be located andsince in many practicable systems as, for example, a system comprising aloop antenna, the peak or nose of the lobe is fairly blunt or broad, itis necessary for accurate determination of the bearing or direction of areflective target, to move or wobble the blunt loke peak across thetarget for the purpose of determining the two angular positions ordirections of the lobe axis at which the intensities of the successivelyreceived reflected waves are equal and smaller than the maximumintensity of the received reflected wave. The point or angular directionequidistant from the two above-mentioned angular directions coincides,approximately, with the actual target direction.

In order to determine more accurately the direction of the incomingwave, single-plane lobeswitching, dual-plane lobe-switching and so-`called conical scanning systems have been proposed. As is known, in thelobe switching and conical scanning systems, the side or slope, ratherthan the peak or nose, of the lobe, is utilized in ascertaining thetarget direction. Along the slope, the intensity of the received or echowave varies rapidly with 'small changes in the angular direction of theincoming wave whereas, as discussed above, along the top or nose of thelobe it varies slowly with the angular changes.

In more detail, in the conical scanning systern, the longitudinal axisof the lobe is displaced by a known angular amount from the longitudinalaxis of a parabolic reflector and is rotated about this latter axis.This rotation of the lobe axis generates in space a cone having its axiscoincident with the reflector axis, the lobe having a so-calledequi-intensity point on one of its slopes coinciding with this axis ofrotation. When, with this system', the reflecting target is on thelongitudinal axis of the reflector, the point of intersection of thetarget direction and the slope of the lobe remain fixed, and thereceived reflected energy will not vary, as the lobe rotates. degree ormore from this axis,the intersecting point moves along the lobe slopeand the amplitude of the reflected signal will vary, increasing as therotating lobe axis approaches the target direction and decreasing as itswings to the opposite side of the axis. This produces an amplitudemodulation of the reflected signal, the amplitude of which gives someindication of the amount of displacement and the phase of which is anindex of the direction in which the longitudinal axis must be moved tobring it into alignm'ent with the reflecting target. To facilitatedetermination of the phase of this modulation, a small two-phasesinusoidal generator is drivenA in synchronism with the rotation of thebeam and the voltages derived from the generator are used as phasereference standards. Visual indication of the targets vlocation withrespect to the axis of the reflector is obtained from a presentation' ofthe pip or spot of reflected energy on a' cathode-ray tube screen.L Itis obvious that any factor other than a reflecting object that causesamplitude modulation will cause lsuch a system' to give a falsedirection indication.

The copending application of Phillip H. Smith, Serial No. 498,622, filedAugust 14, 1943, discloses one type of conical scanning system such asdescribed above. In this system a sem'idipole radiating antenna isvrotated in ront of a paraboloidal reflector.

ally occurs in the antenna load impedance as it is rotated to cause themagnetron Aoscillator to correspondingly change its frequency or poweroutput. As previously stated, this change in os- However, if the objectis a fraction of a Experience therewith has; demonstrated that suiiicient variation occasioncillating frequency results in frequencymodulation in the transmitted and reflected signals which modulation ischanged to amplitude modulation in the received signal by the frequencydiscrimination of the lter circuits of the receiver. This action whencombined with a signal reflected by a target results in:a false'direction indication of the target.

In addition to frequency excursions, the cyclic variations in theantenna load impedance give rise to cyclic variations in the energyoutput. These power variations are indistinguishable in the receivingequipment from the amplitude modulation caused by the location oi thetarget with respect to the reference axis'and accordingly contribute anerroneous component to the indicated location. V

It is accordingly one of the objects of'this invention to control thevariation in the impedance of al load which forms a portion of anelectrical. Isystem comprising a source and a connected load,the'ini'pedance of whichwould vary if it were not controlled.

It isanotherpbject 'of the invention to reduce to a mini-mum theimpedance 'variations inV an antenna 'system employing an antennaelement in moving relationship with adjacent surfaces.

The iabove objects areaccomplished through 'the use vofsimplainexpensive, adjustable means which may be -readjusted tocompensate for changed operating conditions if circumstances indicatethe desirabilityeof-'so doing.

`-The manner in which 'the Ainvention accomplishesthe'se objectsfand thevarious features attendantth'ereupon may lbemore Vclearly appreciate'dwhen f it is considered i Iin connection with one preferred embodimentasshown in the Vattached 'drawing inv-which: Y

fFig. "'1 shows 'a longitudinal. cross-section of a directive'antennasyste-m embodying lone aspect offtheinvention; A

'Fig'. i 2 shows iin "detail a longitudinalcross-section-offthe.support-bearing forthe outer end of the f'antennaline offFi'g. 11; and

SF'igfS :shows across-sectional end View of the shaft' supportof vfFig.'2.

*Experience vhas Yin'dica'ted that the variations inithe load impedancearise from one or more origins. One of these is :in-the mechanicalirregularities yproduced in manufacturing processes which irregularitieslar-"ewithin the lbest practicable manufacturing tolerances. Another isin variations @permissible `ein y the 'assembling jand mountingv:processes of. a complete structure. A third.: arises from the rnecessarily unsymmetrical arrangement fof unrelated surfaces ineffective close proximity Pto :the apparatus. Variations arising vvfrom'these andjpossibly4 other sources are virtually eeliminatedthrough theuse -of this'in-Y vention.

:Referringeto f'ltig.v 1, reference `numeral Ii! `denotes aparaboloidalfreector or secondaryfantenname-ving favertex: H,-faprincipal'- axis "I2 and a focal plane Y I 3. NumeraL` I 4 -denotesla coaxialline comprising aninner rotatablefconductor- I5 and astationary/:outer conductor I 9. The vvinner conductor .I5 extends.through vertex II kand is.

aligned with -axis I2,.-.and it constitutes a shaft driven by Va motorvI'I andsupportedlateach end lay-bearings., l 8. Atquarter wave primaryantenna element located .inthe focal-plane I3 :of the Y reflector .unitI 0 and has its axis substantiallyV rpendicularjto the-.axis I2 fofthereectorunit and to the axisof theinner4 rotatingconductor l5.The.antenna. .'20 is connected .to the rotating faces approaches zero. Av of the main coaxial line is connected to the 'fand 24 of these linesare coupled. through a quar- ,fter kwave coupling.

This coupling comprises a quarter wave sleeve 2'! and a correspondingtubu- Illar portion 'of the shaft i5 as viewed from either end ofthe'li-ne. fface'ofh'einner conductor i5 and the inner sur- "face `ofthe sl'eeveZ'i is exceedingly large since the :structure is ga-.quarterWavelength long and The capacity between the surtherefore the impedancebetween these two sur- The inner conductor 24 sleeve vmember il? `andtherefore to the rotating Ishaftilthrough the extremely lou7 impedance'existing'b'etween thesieeve 2l' and the shaft I5. A coaxial lineZcomprising an inner conductor 29 and an outer conductor it forms acoaxial v tuning stub and is connected to line itat a point opposite`line 23. rEhe impedance of the primary antennaZii is matched to that ofthe line I4 through a pair-of serially connected quarter wave impedancetransformers 3i and 32. Transformers 3x! :and 32 comprise the innersurfaces ofuuarterwavelength tubular conductors 'irl ande-t',respectively, -andfcorresponding portions ci the rotating shaft or innervconductor 'i 5. .The Vr'iare'd quarter wave tubular Yconductorconstitutes Whatris commonly called a Venturi. Numeral''l designates `areflectingwantenna which rotates'in unison withfthe `selnidipole z2@ andWhich Ahas-rior its -purpose the Vreflection of electromagneticWavesl-fromthe sernidipoie antenna Ztl to the paraboloidal'reector I-.This-reflecting antenna 3l', :the primary antenna 2i! and the Venturistructure-33am in a-.fPlexiglas housing 35=which is-.attached to thecoaxial lline iii, and-functions asa support for the bearing housing 36and end cap 113. f

shown `Yin/greaterdeail in Figs. .v2 and''the bearing housing --and endcap support and enclosethe-coaxial inner rotating conductor t5. Thevvinner.conductor l5 is supported by androtates with the innerrow lofthe loa-ll type bearing.V

I8 which isrnounted inside of a double eccentric comprising units itYand 39. These inner'and outereccentric units Stand 3S are enclosed inthe .cylindrical unit .d2 andheldin placerby the locking nutrorlockingring dil. .Numeral il designates av cap screw vtting into a threadedreceptacle inside .oi the` inner eccentric .unit *et and encloses` theend of the rotating winner l.conductor i5. Eccentrics iand containtheslots ,or redresses-i6` and .4?, respectiveiy, which are designed toreceive the tool used inpositioning,oneeccentric withrespectto theother. vAs shown Figf, the. outer eccentric's@ contains the lcciingsetscrews 44 and 45 for locking in positionthe inner eccentric $8 Vin anydesired relatieM to the outer eccentric 3S.

r-Inconsidering the operation of this embodi ment of .the invention,reference .should be made toourcontending-.application SerialNo67.6t'i16'J V ledJunerll, 1946, which sets iorthindetail .the

method of ymeasuring the variation` in the .loadl deter;

impedanceof .the antenna unit .2li Vand `the minationofthefpropencorrection therefor. f A As .previously discussed, thefrequency .modue lation imparted tothe transmitted wave may, in thefilter circuits incorporated in the translation device22,.become=converted to amplitude modulation :and result in shifting ofthe spot which denotesl the reecting objects location as seen on aAcathode-raytube screen (not shown) incorporated-inthe radar transceiverdevice 22. Because- 'of structural irregularities, the quarter waveantenna 20 and the rotating inner conductorIl l5 do not maintainasymmetricaldisposition with respect. to allV nearby objects and surfacesas-the antenna is rotated about the axis of the reflector unit Il). Asseen from the magnetron oscillator unit included in the radartransceiver device 22, any variation in the impedance of the antennaunitfappears as a varying load on its output. It has been determinedthat the primary,T sources of impedance variation arise throughproximity variations between the inner rotating conductor i5 and theinner surface of the outer conductor i9Y of the coaxial line Mv, betweenthe inner conductor I5 and the quarter wave elements 34 and 34 of thetransformers 3| and 32, between the outer surface of the conical memberfil and the inner surface of the Venturi member 33, between the quarterwave 'antenna element 2G and the lip "or'outer-edge of the-Venturimember 33, all'of which occur as the vquarter wave antenna element 2D isrotated about the reflector axis i2. In addition to the foregoing,variations in the impedance of the quarter wave antenna 2G occur as thisantenna rotates and changes its proximity to nearby objects such, forexample, as deck structures, masts, rigging or gun mounting on ships.Variations of this latter character are more or less continuouslychanging, but through the use of this invention they may becounterbalanced for a given position of the antenna system. These lattervariations are minor and do not materially affect the oscillatoroperation. Variations of the former class are more or less permanent intheir nature and arise from lack of symmetry between the surfaces of theantenna system. This lack of symmetry results in a variation in the loadimpedance as the antenna element rotates; and this variation alternatelyor cyclically loads the magnetron oscillator to a greater or lesserdegree and results in cyclic frequency or power variations in theoscillator output. The frequency Variations are equivalent to cyclicfrequency modulations of the transmitted microwaves, at the antennarotating frequency, and when reflected energy so modulated is receivedwith the normally amplitude modulated refiected energy from a target, itwill be converted to amplitude modulated components in the receivingunit and will cause an error in the indicated location of the target. Aspreviously stated, the cyclic power output variations areindistinguishable in the receiving apparatus from amplitude modulationsimparted by the target.

The effect of this lack of symmetry during the rotational cycle may beminimized or eliminated by dynamically adjusting the antenna systemimpedance through the use of the inner and outer eccentrics 38 and 39 ofFigs. 2 and 3. This adjustment is accomplished by manually rotating theinner or outer eccentrics 38 and 39, or both, in a manner such as toshift the bearing I3 supporting the outer end of the rotating innerconductor i5, and therefore such as to shift the axis of rotation ofthis conductor and the associated antenna element 20 to a position wherethe eccentrics maintain substantial symmetry with re- Cil spect to1 theadjacent surfaces duringgthejrotational'cycles. Y ,Y .l z Thedetermination that variations in the impedance matching do occur duringthe rotational cycle and the determination of the magnitude and phase ofthe correction to be applied through the use of the inner and outereccentrics 38 and 39 may be made throughv the use of suitable testingand indicating apparatus. Our copending application mentioned abovefully discloses one type of testing and indicating apparatus and themethod of its use which we have found to be satisfactory for thispurpose.

In general this apparatus provides suitable means for selecting theportion of the transmitted energy that is reiiected from the point ofthe impedance mismatch back toward the transmitting source. Theamplitude of this reiiected energywill vary as the impedance mismatchvaries in itscyclic mode, which variation maybe detected as amplitudemodulation. Through the use of limiting and phase inversion means, thisamplitude modul-ation component is utilized in a phase and amplitudesensitive circuit along with the voltages derived from a small two-phasegenerator that is rotated in synchronism with the antenna element 2li togive on the screen of a cath-r ode-ray tube a visual indication of thevariations in the impedance mismatch as the antenna element is rotated.Observations of this visual indication are used to direct thepositioning of the eccentrics 38 and 39 to eliminate or minimize thesevariations. After suitable adjustment of the double eccentric has beenmade the inner eccentric 38 is permanently positioned inside of theouter eccentric 39 by the locking set screws 44 and 45 of Fig. 3. Theouter eccentric 39 is maintained in its position by tightening thelocking ring 40 in its threaded position in the cylindrical unit 42 asshown on Figs. 2 and 3. The antenna end cap 43 forms a weather-tighthousing for the entire assembly.

As previously stated the antenna impedance also tends to vary because ofunsymrnetrical proximity with adjacent non-related structures during therotational cycle. For any one particular position of the antenna systemat the time of the foregoing adjustments these variations arecompensated. When the antenna system is changed from the position itoccupied during the adjustment period, these latter factors Will againbe uncompensated. However, their effect is in general so small as to benegligible.

Although the invention has been described in connection with a specificembodiment, it is not to be limited thereto inasmuch as other apparatusmay be utilized in successfully practicing the invention.

What is claimed is:

l. In a radio system comprising a source of electric energy, an antennaelement, a line comprising an inner and outer conductor connecting saidsource to said antenna, said antenna being attached to said innerconductor, means attached to said antenna element for imparting rotarymotion thereto, and means for reducing to a substantial minimum theimpedance variation in said rotating antenna, said means comprising adouble eccentric mounting at one end of said rotating inner conductorfor moving the axis of rotation of said inner conductor and said antennaclement with respect to adjacent surfaces.

2. In a radio system comprising a source of electric wave energy, anantenna element, a transmission line comprising an inner'conductor andan `outer eoniiueteiand leenneetine sain source and said` antenna. saidantenna leine'nt being onneetedto saiinner feendueter at a peintadjacent .one end of said conductor, means enneeted te the vother end ofIsaid inner con- 'to said antenna and a double eccentric connected tosaiii inner conductor at said yfirsteinefi'tiened point rer moving saidinner cnduete relative to said enter conductor and fnainta-imng' at asubstantiai vfrninii'n'ufrn yme impedance variation iin said antennaelement.

3. A transmission system comprising 'a Source of electric A e'ry, ac'aiial transmission line connected theiet'o and n'aris'ing rotatableiiiner conductor and a stationary outer conductor, a quarter waveantenna element rigidly attaehed to Vsaid inner conductor, meansenneeted to said inner eonduetor rer rotatinfg said inner conductor andsaid element, and means attached to said rotata-bie inner conductorrerfmaintainine at a sua stantiai minimum the variations in the inputimpedanc'e "oi said antenna element occurring during rotation of saidelement and fs'a'id inner ''oaxial connecter, said inet-ns 'cnmrisingadoubi'e eccentric-'assembly rfo'r' moving the airis ef rotation of said4rotatable inner conductor relative to Ysaid stationary outer contietor.

4. In a transmission system comprising. 2 si. source of electric energy,a transmission line con nected thereto and having an inner rotatableconiductor, a stationary outer conductor, and atleast one impedance matchingtransformer Vincluded in said line, an antenna elementattached to saidinner rotatable conductor, means attached :to said inner conductor forreducing the Variations in the impedance presented to said source whensaid inner conductor is rotated, said means compris` ing a doubleeccentric for changing the axis :of rotation of said inner conductorwhereby var1ations in the transformation ratios of said im pedancematching transformers are reduced.

GEORGE H BAKER ELMO E. CRUMP.

REFERENCES CITED The following references are 'of record in the le ofthis patent:

UNITED STATES PAINTS Briggs et al Dec. .'17, 1%546

